If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
You can use "Disable all add-ons" on the Safe mode start window.
You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")

Start Firefox in [[Safe Mode]] to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).
See [[Troubleshooting extensions and themes]] and [[Troubleshooting plugins]]
If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
You can use "Disable all add-ons" on the ''Safe mode'' start window.
You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")